Which enzyme relieves torsional strain ahead of the replication fork?

Which enzyme relieves torsional strain ahead of the replication fork?

• A. Helicase
• B. Primase
• C. Ligase
• D. Topoisomerase

Answer: (D)

Torsional strain ahead of the replication fork arises because the DNA double helix must be unwound as replication proceeds. The enzyme that relieves this strain is topoisomerase. It temporarily cuts one (type I) or both (type II) strands of DNA, allowing the helix to unwind or to pass another segment through the break, and then reseals the breaks. This relieves the supercoiling that would otherwise stall the fork and disrupt synthesis. Primase makes RNA primers for DNA polymerase to start synthesis, and helicase unwinds the DNA but does not relieve torsional strain by itself. Ligase seals nicks after Okazaki fragment synthesis. So topoisomerase is the key enzyme that manages the torsional topological stress during replication.

Torsional strain ahead of the replication fork arises because the DNA double helix must be unwound as replication proceeds. The enzyme that relieves this strain is topoisomerase. It temporarily cuts one (type I) or both (type II) strands of DNA, allowing the helix to unwind or to pass another segment through the break, and then reseals the breaks. This relieves the supercoiling that would otherwise stall the fork and disrupt synthesis.

Primase makes RNA primers for DNA polymerase to start synthesis, and helicase unwinds the DNA but does not relieve torsional strain by itself. Ligase seals nicks after Okazaki fragment synthesis. So topoisomerase is the key enzyme that manages the torsional topological stress during replication.